INFLUENCE OF VISUAL AND SOMATOSENSORY INPUT ON LEG EMG RESPONSES IN DYNAMIC POSTUROGRAPHY IN NORMALS

The contribution of visual and somatosensory input to stabilization af ter sudden postural disturbances was investigated. Fast transient plat form movements (50 degrees/sec, 4 degrees), rotating toe-up around the ankle-joint, evoke EMG responses in the triceps surae (TS) and anteri or tibial (TA) muscles. The short and medium latency responses (SL, ML ) in the TS destabilize, the long latency responses (LL) in the TA sta bilize the upright posture. The influence of vision was tested in norm als comparing the conditions eyes open with eyes closed. The influence of additional somatosensory input was tested with the operator's inde x finger touching the back of the subject with eyes closed. Absent vis ual input (eyes closed) resulted in a decrease in latency of the stabi lizing LL response and an increase in the integrated EMG (iEMG). With eyes closed the latency of the LL response decreased by 4 msec on aver age (mean eyes open: 111.6 msec (S.D. = 17.3), eyes closed: 107.6 msec (S.D. = 17.5); P < 0.001 (Wilcoxon test)), the iEMG increased by 10% (mean eyes open: 13.9 mu V.Sec (S.D. = 7.6), eyes closed: 15.5 mu V.se c (S.D. = 8.6); P < 0.001). Additional somatosensory input as an exter nal reference for body orientation in space resulted in an increase in latency and decrease in iEMG: the latency increased by 7 msec on aver age (mean eyes closed: 107.6 msec (S.D. = 17.5), index finger and eyes closed: 114.7 msec (S.D. = 19.9); P < 0.001), the iEMG decreased by 2 0% (mean eyes closed 15.5 mu V.sec (S.D. = 8.6), index finger and eyes closed: 12.3 mu V.sec (S.D. = 7.5); P < 0.001). There was no signific ant change in latency of SL and ML. We conclude that postural EMG resp onses in leg muscles after fast transient platform movements change ac cording to different functional demands, modulated via visual and soma tosensory input.